The present invention relates to a speech decoder for properly transmitting and decoding speech signals through a transmission path, e.g., a radio transmission path, in which errors tend to occur.
As a method for coding speech signals at low bit rates of about 8 to 4.8 kb/s, a CELP (Code Excited LPC Coding) method is known, which is described in, e.g., M. Schroeder and B. Atal, "Code-excited linear prediction: High-quality speech at very low bit rates" Proc. ICASSP, pp. 937-940, 1985 (reference 1). According to this method, on the transmission side, a spectrum parameter representing the spectrum characteristics of a speech signal is extracted from a speech signal of each frame (e.g., 20 ms). The frame is divided into subframes (e.g., 5 ms), and a pitch parameter representing a long-term correlation (pitch correlation) is extracted from a past excitation signal in units of subframes. Long-term prediction of speech signals in the subframes is performed using the pitch parameter to obtain a difference signal. For the difference signal obtained by long-term prediction, one type of noise signal is selected so as to minimize the differential power between the speech signal and a signal synthesized by a signal selected from a codebook constituted by predetermined types of noise signals. In addition, an optimal gain is calculated. Subsequently, an index representing the type of selected noise signal and the gain are transmitted together with the spectrum parameter and the pitch parameter.
On the reception side, speech is reproduced using the received parameters.
In the conventional method in reference 1 described above, when no errors are present in a transmission path, high-quality speech signals can be reproduced. If, however, transmission path errors frequently occur as in a radio transmission path, the sound quality of reproduced speech is considerably deteriorated. As an example, in a mobile telephone, burst errors tend to occur due to fading and the like, and the bit error rate is as large as 1% to 3%, thus greatly deteriorating the quality of reproduced speech. In such a transmission path in which errors tend to occur, a forward error correction code (FEC) is used to protect the transmitted parameters. Furthermore, in order to deal with burst errors, an error detection code (e.g., a CRC code) is used. This error detection code serves to detect an error, which exceeds the correction capacity of the FEC, on the reception side. These error correction codes are described in detail in, e.g., Hirata et al., "Application of Forward Error Correction and Voice Coding Techniques to Satellite Communication Systems : Current Trend and Future Technical Problems", Material for Theoretical Research Meeting of the Institute of Electronics and Communication Engineers of Japan, IT84-30, pp. 1-8, 1984 (reference 2).
In the conventional speech decoding method, when an error exceeding the correction capacity of the FEC is detected by an error detection code on the reception side, the received parameter of a frame in which the error is detected is replaced with a parameter properly received in the past. More specifically, in such a frame, speech is decoded by repeatedly using a parameter properly received in the past. Such a decoding method is described in detail in, e.g., R. V. Cox et al., "Robust CELP Coders for Noisy Backgrounds and Noisy Channels", Proc. ICASSP89, pp. 739-742, 1989 (reference 3). Although the speech decoding method described in reference 3 is simple, since it repeatedly uses a past parameter, the sound quality of speech may deteriorate or abnormal sounds may be generated in frames in which the characteristics of speech signals are changed, especially at transitions, due to the repetitive use of the parameter.